-LRB- CNN -RRB- -- Rwanda held solemn commemorations Monday to mark the 20th anniversary of the genocide in Rwanda , with many survivors overcome with emotion as they relived the trauma of the killings that left 800,000 people dead .

Official mourning , which began three months ago with a flame of remembrance touring towns and villages across the small central African nation , culminated Monday with the arrival of the torch at the national genocide memorial .

Wreaths were also laid in Kigali 's soccer stadium , where U.N. Secretary General Ban Ki-moon , heads of state and European and U.S. diplomats gathered .

President Paul Kagame lit a flame that will burn for 100 days , the length of time the killings lasted .

At a ceremony at the national stadium , survivors of the genocide recounted their memories of the killings and of their survival . Some , sobbing and overcome with emotion , were carried outside .

`` This should never happen in human history , '' Ban said of the massacre .

Diplomatic dispute

The events also bore reminders of festering anger as a diplomatic dispute broke out over renewed allegations from Kigali of French complicity in the genocide .

France said Monday that its ambassador to Rwanda had been barred from attending the events marking the anniversary .

In a written statement , a spokesman for the French Foreign Ministry said Ambassador Michel Flesch , who was to represent France during the commemorative events in the Rwandan capital , had been informed Sunday night `` of the decision of the Rwandan authorities not to authorize him to take part in the ceremonies . ''

`` We regret this decision . However our embassies will take part today in the several ceremonies held across the world to pay tribute to the memory of the victims of the genocide , '' the spokesman added .

France said over the weekend that it had canceled plans to attend the ceremony after Kagame 's accusations of French involvement in the massacre .

The weekly Jeune Afrique quoted Kagame as saying in an interview that both France and Belgium had played a `` direct role ... in the political preparation of genocide and participation in its execution . ''

The French Foreign Ministry said that France was `` surprised by the recent accusations made by the Rwandan president '' and that Justice Minister Christiane Taubira , who had been due to travel to Kigali on Monday , would no longer attend the commemoration .

`` These accusations are in contradiction with the process of dialogue and reconciliation that has gone on for several years between our two countries , '' the ministry 's statement said .

Search for closure

As the 20 years are marked , many in Rwanda still are in search of closure .

Niyonsenga Erick Rafiki , 24 , says he ca n't remember the night his father was killed . But he searched for answers over the years and eventually found his father 's killer .

The killer , Sekamana Musa , served 10 years in prison for that murder and other crimes during the genocide . Musa said he was released after confessing .

`` When I came out , I found the people we had committed crimes against , be it the mother of this boy -LRB- Rafiki -RRB- or this boy himself , '' Musa said .

Musa has promised to help Rafiki find his father 's body . Recently , the two men and others went to a spot where Musa believes Rafiki 's father was buried . As Rafiki watched , Musa and other men dug , hoping to find the remains so that Rafiki can give him a proper burial .

`` I never had a chance to bury my parent , as his child , in a decent way , '' Rafiki said .

The body was n't immediately found , but they said they would continue looking .

Three-month killing spree

In 1994 , Hutu extremists in Rwanda targeted minority ethnic Tutsis and moderate Hutus in a three-month killing spree that left an estimated 800,000 people dead .

Hutu attackers burned down churches with hundreds or thousands of Tutsis inside . In some cases , wielding machetes , they entered homes and slaughtered children and grandparents .

The violence was triggered by the death of President Juvenal Habyarimana , an ethnic Hutu , in a plane crash April 6 , 1994 .

The armed forces of Kagame 's Rwandan Patriotic Front eventually defeated the troops , stopping the wave of bloodletting .

France , an ally of the Hutu nationalist government before the 1994 killings , has repeatedly denied any complicity in the genocide . Rwanda broke off diplomatic ties with France in 2006 , and relations were restored only three years later . Rwanda was a Belgian colony until 1962 .

Last month , a Paris court sentenced former Rwandan spy chief Pascal Simbikangwa to 25 years in jail for his role in the genocide , the first such conviction in France .

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CNN 's Nima Elbagir , Laura Akhoun and Nana Karikari-apau contributed to this report .

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Solemn commemorations mark 20 years since genocide began

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France canceled plans over weekend to send minister to Kigali on Monday

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This followed accusations of French involvement in the massacre by Rwanda president

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In 1994 , a three-month killing spree left an estimated 800,000 people dead